1. Field of the Invention
The present invention relates to a passive optical network, and more particularly to a method for implementing various functions in a Gigabit Ethernet-Passive Optical Network (GE-PON) system and a structure of an Ethernet frame employed in same.
2. Description of the Related Art
To configure an optical subscriber network from a central office to a building or general home, a variety of network architectures such as an x-Digital Subscriber Line (xDSL), Hybrid Fiber Coax (HFC), Fiber To The Building (FTTB), Fiber To The Curb (FTTB), Fiber To The Home (FTTH), etc. have been proposed. Among these network architectures, an FTTx (x=B, C or H) can be divided into an active FTTx based on a configuration of an AON (Active Optical Network) and a passive FTTx based on a configuration of a PON (Passive Optical Network). Because the PON is a network configuration having a point-to-multipoint topology, it is considered to be an optical subscriber network, which will be economical in the future.
The PON is the optical subscriber network forming a distributed topology having a tree structure in which one Optical Line Termination (OLT) and a plurality of Optical Network Units (ONU) are coupled via a “1×N” passive optical splitter. Recently, in an International Telecommunication Union—Telecommunication section (ITU-T) standard, an Asynchronous Transfer Mode-Passive Optical Network (ATM-PON) system based on a point-to-multipoint connection has been defined in documents relating to an ITU-T G.982, ITU-T G.983.1 and ITU-T G.983.3. Further, standardization for a GE-PON system based on a point- to-multipoint connection is in progress in Institute of Electrical and Electronics Engineers (IEEE) 802.3ah TF.
The Gigabit Ethernet based on the point-to-point connection and a technology associated with a Medium Access Control (MAC) for the ATM-PON based on the point-to-multipoint connection have already been standardized, see IEEE 802.3z and ITU-T G.983.1. For example, U.S. Pat. No. 5,978,374, entitled “PROTOCOL FOR DATA COMMUNICATION OVER A POINT-TO-MULTIPOINT PASSIVE OPTICAL NETWORK”, issued Nov. 2, 1999, to Ghaibeh, et al. discloses the technology associated with the MAC for the ATM-PON in detail.
A structure of a standard frame for the Gigabit Ethernet based on the conventional point-to-point connection standardized is shown in FIG. 1. A schematic configuration of a standardized ATM-PON system is shown in FIG. 2. In the standardized Gigabit Ethernet based on the point-to-point connection, various functions are not defined that are required in a PON system based on the point-to-multipoint connection. The ATM-PON has been standardized for the first time as a form of the PON based on the point-to-multipoint connection.
As shown in FIG. 2, the standard ATM-PON system includes one OLT 10, that is located at the root of a tree structure and that provides subscribers of an access network with information. The OLT 10 is coupled to an ODN Optical Distribution Network (ODN) 16 having a tree topology The ODN 16 acts to distribute downstream data frames received from the OLT 10 and multiplexes upstream data frames to transmit the multiplexed data frames to the OLT 10. A plurality of ONUs 12a, 12b and 12c receive the downstream data frames from the ODN 16 to provide termination users 14a, 14b and 14c with the received downstream data frames. The ONUs 12a, 12b and 12c transmit the upstream data frames from the termination users 14a, 14b and 14c to the ODN 16. In FIG. 2, the termination users 14a, 14b and 14c include a variety of termination devices, which can be used in the PON including a Network Terminal (NT).
The ATM-PON system as shown in FIG. 2 performs downstream or upstream packet transmission of ATM cells in the form of a frame having a constant size of 53 bytes. In the PON having the tree structure shown in FIG. 2, the OLT 10 inserts downstream ATM cells into downstream frames to be distributed to the ONUs 12a, 12b and 12c. In the upstream transmission, the OLT 10 accesses upstream data received from the ONUs 12a, 12b and 12c on the basis of Time Division Multiplexing (TDM). At this time, the ODN 16 connected between the OLT 10 and the ONUs 12a, 12b and 12c is a passive device. Accordingly, the OLT 10 avoids causing data collision in the passive device of the ODN 16 by employing a virtual distance correction using a ranging algorithm. Further, when the OLT 10 transmits downstream data to the ONUs 12a, 12b and 12c, the OLT 10 and the ONUs 12a, 12b and 12c exchange a secret encryption key for security and an Operations, Administration and Maintenance (OAM) message. A corresponding data field is provided within a dedicated or general ATM cell of an upstream/downstream frame so that the OAM message can be exchanged at a predetermined time interval.
As described above, the ATM-PON system configures upstream and downstream frames on the basis of a constant size ATM cell. The ATM-PON system employs TDM for upstream packet transmission in a tree structure for a point-to-multipoint connection.
As Internet techniques have evolved, subscriber sides have needed greater bandwidth, and, for this reason, a Gigabit Ethernet system is preferred over an ATM system. In order to obtain greater bandwidth, relatively costly equipment is needed in an ATM system and only provides limited bandwidth improvement (e.g., 622 Mbps). Further, an ATM system must segment IP (Internet Protocol) packets. By contrast, relatively inexpensive equipment can be employed in a Gigabit Ethernet system and higher bandwidth (e.g., 1 Gbps) can be provided for end-to-end transmission. Accordingly, the Gigabit Ethernet system is preferred over the ATM system for the PON architecture of a subscriber network.
In the Gigabit Ethernet, a MAC protocol based on point-to-point connection and a collision avoidance scheme have been standardized and a MAC controller chip is commonly used. However, standardization for a GE-PON architecture including the MAC protocol is currently in progress. Further, the point-to-multipoint connection is not implemented by the GE-PON and a format for a frame to be used between an OLT and an ONU in the GE-PON has not been defined.